Melt butter in a medium saucepan over medium-low heat until it begins to foam, 15 to 20 minutes.

While butter is melting, place egg yolks, lemon juice, salt, pepper, and cayenne into the pitcher of a blender. Cover and pulse on HIGH speed for 2 seconds.

Once butter is hot and foamy, with the motor still running on HIGH, gradually add melted butter in a slow, steady stream through hole in blender lid (I always have a towel at hand to help cover the plug hole to keep the butter from flying everywhere), leaving milky solids behind. Season with salt and pepper.